Laminating systems for applying two sheets of plastic film on opposite surfaces of an article have been known for a number of years. Examples of such systems are disclosed in U.S. Pat. Nos. 3,309,983; 4,268,345; and, 4,090,911. In all of these systems, a pair of supply rolls supply two continuous webs of film between and into the nip of a pair of pressure laminating rolls which also receive the article therebetween.
The respective webs of laminating film have a heat-activated coating on one surface thereof and the system includes shoe-type heating members that are partially wrapped around the laminating rollers to heat the rollers. The respective webs of film are entrained over the arcuate shoes to heat the film sufficiently to activate the adhesive so that it becomes slightly fluent or tacky and the pressure laminating rollers bond the heated webs to the article, which is sandwiched therebetween. In this type of system, two sets of pull rollers are located downstream of the pressure rollers and maintain tension on the laminated article as it is being moved between cooling fans, located above and/or below the laminated article.
The shoe-type heating members usually are heated through electrical resisting heating elements that are either mounted in the shoes or adjacent the rear surface thereof. The shoes define generally arcuate surfaces over which the laminating film passes to provide extended contact with the film immediately prior to entering the nip of the laminating rollers and thus activate the adhesive.
Certain problems have been encountered with laminating systems of this type and the primary shortcoming is the unevenness of the heat produced by the shoe-type member which results in varying temperatures across the entire width of the web. In addition, the partially-heated film immediately enters the laminating area from the heated shoes and does not allow for adequate transfer of the heat from the sheets to the adhesive film and also does not allow for distribution of the heat across the sheets. It will be appreciated that uneven heat applied across the web of the sheet can produce either overheating or underheating of the web, which will result in a defective product.
Another problem encountered with the prior art systems is the fact that the laminating film must be of a certain character and, thus, the number of different laminating films that can be used is limited. For example, in the systems disclosed in the prior art, all of the heating of the laminating film is done by the stationary heated shoes immediately before the laminating film enters the nip between the pressure rollers. This is necessary to prevent stretching or elongation of the film in the heated condition, which may cause the film to wrinkle. This is particularly true when using a film such as polypropylene and, thus, these types of machines are incapable of laminating articles utilizing films other than polyester.
Thus, prior art laminating is effectively limited to using polyester films which can be heated by the stationary heated shoes of the type disclosed in U.S. Pat. No. 4,268,345 without stretching or wrinkling.
A further problem with the prior art types of machines is the fact that the speed is extremely limited since the film must be in contact with the heating shoes a sufficient amount of time to elevate the temperature thereof sufficiently to activate the adhesive. Thus, it is necessary to limit the speed of operation to obtain the proper heating characteristics, which may vary depending upon the adhesive and the type of polyester film that are being utilized.
U.S. Pat. No. 4,743,334, issued to Karl Singer, discloses an improved laminating system over the type disclosed in U.S. Pat. No. 4,268,345. The Singer patent, assigned to the Assignee of the present invention, discloses a laminating system that is capable of effectively laminating a variety of different films and includes a pair of freely-rotating preheat rollers that are juxtaposed between the supply rollers and the pressure laminating rollers. The preheat rollers are heated to a temperature slightly below the activating temperature of the adhesive film and laminating film. The adhesive film is elevated to the activating temperature for the adhesive film by the laminating rollers and is bonded to the article.
The drive mechanism for the Singer laminating system includes a drive motor that drives the lower pressure laminating roller and the lower first puller roller, while the second lower pull roller is driven from the first pull roller. The first pull roller is driven at a speed slightly greater that the speed of the laminating roller to maintain a taut condition for the laminated article while it is being cooled by cooling fans.
While the Singer laminating system has met with a remarkable degree of commercial success, certain shortcomings have been observed which are also encountered in other current commercial double-laminating systems. Customarily, the article to be laminated has a width that is less than the width of the laminating film and a certain amount of wrinkling has been encountered in the film that is located beyond opposte edges of the bonded article. While most of this wrinkled excess film is trimmed from the article, the finished product may still have wrinkles along the side borders, which is undesirable.
Further, because of imperfections in either the film or the article being laminated, the lamination rollers may contact the film for an extended period beyond that which is desirable. This may cause non-uniformity in the final laminate product. Additionally, such imperfections can cause the final laminate to pass through the machine at an uneven rate and with fluctuating tensions, which is also undesirable because it can negatively affect the uniformity of the final product.